Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) are all pressure-driven separation processes allowing a broad range of neutral or ionic molecules to be removed from fluids. Microfiltration is used for removal of suspended particles greater than 0.1 microns. Ultrafiltration commonly excludes dissolved molecules greater than 5,000 molecular weight. Nanofiltration membranes have been characterized as those passing at least some salts but having high rejection of organic compounds having molecular weights greater than approximately 200 Daltons. Reverse osmosis membranes have high rejection of almost all species.
While NF and RO are both capable of excluding salts, they typically differ in selectivity. NF membranes commonly pass monovalent ions while maintaining high rejection of divalent ions. By contrast, reverse osmosis membranes are relatively impermeable to almost all ions, including sodium and chlorine ions. Still, a continuum of properties are possible, and NF membranes have sometimes been described as “loose” RO membranes.
One of the first industrially utilized membranes capable of removing dissolved salts from water was the cellulose acetate membrane developed by Loeb and Sourirajan (U.S. Pat. Nos. 3,133,132 and 3,133,137). This type of membrane is prepared by a phase inversion process resulting in an asymmetric structure. Selectivity results from a thin discriminating layer that is supported on a thicker, more porous layer of the same material. While versions of this membrane still provide adequate performance properties (flux and rejection) for some applications, cellulose acetate membranes have the noted problem of being prone to hydrolysis in either very high or very low pH solutions.
An alternative method of forming membranes capable of rejecting dissolved salts is by interfacial polymerization. This technique can result in a very thin discriminating polymer layer (commonly less than 2000 Å) supported by a layer of a chemically-different material chosen for other properties, such as strength. The thin discriminating layer can provide high selectivity while offering little resistance to flow. The interfacial polymerization is most commonly performed as a polycondensation between amines and either acid chlorides or isocyanates.
Owing to the wide variety of monomers that are available by this method, many membranes have been made with advantageous combinations of flux, rejection, and stability to high and low pH. Two of the most commercially successful of these membranes have resulted from the reaction of trimesic acid chloride (TMC) with either piperazine or meta-phenylenediamine (MPD), as described by Cadotte (U.S. Pat. No. 4,259,183 and U.S. Pat. No. 4,277,344 both of which are incorporated herein by reference). FilmTec FT30 and FilmTec NF40 are prototypical membranes made by polycondensation of TMC with MPD and piperazine, respectively. Other membranes made with piperazine or substituted piperazine have been described in the literature, e.g. see U.S. Pat. Nos. 3,687,842 and 3,696,031, with a noted advantage of stability to elevated temperature and resistance to hydrolysis at low and high pH.
Interfacial polymerization of the commercial FilmTec NF40 NF membranes was performed according to a process described in the literature, e.g. see J. E. Cadotte, R. S. King, R. J. Majerle, and R. J. Peterson, “Interfacial synthesis in the preparation of reverse osmosis membranes”, J. Macromol. Sci.—Chem., A15 (5), p. 733. A polysulfone substrate was saturated with an amine solution comprising between 0.5% and 3% piperazine in water. The amine solution also contained an equal amount of N,N-dimethylpiperazine used as an acid acceptor. Excess amine was drained off or “squeezed” off by means of a rubber roller, and the polysulfone support was covered with between 0.1 and 1% TMC in freon or other suitable hydrocarbon solvent. The FilmTec commercial membranes NF45 and SR90 are still made by similar processes, with additional proprietary chemicals added to the water and/or organic phase. In each case, more than 60% of incorporated amine in the polyamide is piperazine.
A spiral-wound filter cartridge is one conventional means to incorporate large amounts of RO or NF membrane into a small volume. The construction of spiral wound elements has been described in more detail elsewhere, see for example U.S. Pat. No. 5,538,642 incorporated herein by reference. Such an element can be made by wrapping feed spacer sheets, membrane sheets, and permeate spacer sheets around a perforated permeate tube.
A membrane sheet is interposed between each feed spacer sheet and its adjacent permeate spacer sheets. The membrane sheet is oriented with the discriminating layer facing the feed spacer sheet. Often this arrangement is accomplished by folding the membrane sheet so that its discriminating layer faces in and the two sections on either side of the fold line sandwich a feed spacer sheet. Permeate and feed sides are further isolated by perimeter glue lines that bond adjacent membrane sheets on their support side and enclose each permeate sheet within a three-sided envelope. Feed solution flows axially through the feed spacer and exits on the opposite side as concentrate. Permeate solution passes under pressure through the membrane and is directed to the permeate tube by the permeate carrier sheet.
After construction, or when not in use, the spiral elements must be stored in a manner that does not harm performance of the incorporated membrane. One effective means for storing elements after manufacture is dry storage, where the water is removed in a drying stage. In many cases, dry storage results in excellent stability of performance characteristics (flux and rejection). Dry storage also has advantages in cost and in its inhibition of biogrowth. However, some dry membranes are less stable than their wet counterparts. Also dry storage does not easily allow the option of wet-testing elements prior to shipping, and testing for defective elements must then be performed by other means.
A typical method for storing wet elements before shipping is to contact elements with a preservative solution. Several different solutions have been contemplated, and some examples are described in U.S. Pat. No. 4,293,420, EU 0115375, and other publications, see for example L. H. Rowley, “A screening study of 12 biocides for potential use with cellulose acetate reverse osmosis membranes”, Desalination, 88, (1992), 71–83; The nominal stability of performance properties for a given preservative is seen to vary substantially with membrane type.
A preservative based on sulfiting agents is now most commonly used for elements constructed with interfacially polymerized membrane, including both MPD-based and piperazine-based membranes. Sulfiting agents include sulfur dioxide (SO2) and sulfurous acid (H2SO3), as well as inorganic sulfites such as bisulfite (HSO3−1), sulfite (SO3−2), and metabisulfite (S2O5−2). Sodium bisulfite (NaHSO3) is generally obtained commercially as sodium metabisulfite (Na2S2O5); the interconversion takes place by addition of water. The distribution of sulfurous species in water depends on pH, with H2SO3 dominating at low pH. While the greatest bioactivity is usually associated with large SO2 concentration, both bisulfite and sulfite are reducing agents capable of combining with oxygen. The reaction rate for removal of dissolved oxygen is greatest near neutral pH. See, for example, N. Matsuka, Y. Nakagawa, M. Kurihara, and T. Tonomura, “Reaction kinetics of sodium bisulfite and dissolved oxygen in seawater and their applications to seawater reverse osmosis”, Desalination, 51 (1984), 163–171.
To maintain bioactivity, preservative solutions that react with oxygen, such as those containing sulfiting agents, must be isolated from air. When shipping an element preserved with bisulfite, the industry standard has been to enclose the element within a polymer barrier bag comprising at least one layer of polymer having low permeability to oxygen, such as SARAN. A typical bag may have a 5 mil total thickness and the better oxygen barrier bags would have a permeability of around 10 cm3/m2/day at 68° F. Thicker barrier layers can decrease oxygen passage further, but this results in increased costs and handling problems that have not previously been justified by the demands of the application. Two liters of 1% sodium metabisulfite enclosed within a barrier bag having 0.5 square meters of surface area and an oxygen permeability of 10 cm3/m2/day can maintain a reducing environment for more than a year. These values approximately correspond to those associated with preserving an 8″ diameter spiral wound element.
Despite the long period of time that a reducing environment may be maintained at room temperature, it has been found that storage of elements in this manner can result in pronounced instability of the element's performance properties if the element is subjected to high temperatures (e.g. in excess of 100° F.) during storage. We have discovered that this is particularly the case with elements formed of piperazine-based membranes. It is a purpose of this invention to provide a storage assembly for elements formed of piperazine-based membranes capable of withstanding large excursions in temperature. A further purpose is to provide a storage assembly for such membranes that inhibits bio-growth.